1. Field of the Invention
The present invention is directed to a coating composition which provides a transparent coating having improved abrasion resistance upon curing. More specifically, the coating compositions of the present invention contain a blend of colloidal antimony oxide and colloidal silica to provide filled organo-polysiloxane coatings having superior resistance to abrasion than filled organo-polysiloxane coatings containing only colloidal silica.
2. Description of the Prior Art
The use of transparent synthetic polymeric materials as windows or in glazing for automobiles, buses, aircraft and public buildings has widely increased in recent years. While these transparent polymers, such as polymethylmethacrylate, can be readily fabricated into the desired shape, they exhibit relatively low abrasion resistance. Consequently, to prevent the polymeric material from losing its optical clarity due to abrasion, it is imperative that the polymeric material be coated with a coating composition which is transparent and which protects the surface of the polymeric material from abrasion. Various coating compositions which can be coated on the surface of the transparent polymers are known in the art. In particular, prior coatings such as silica-containing solutions and polysilicic acid fluorinated copolymers are difficult to apply, have a low humidity resistance, or are expensive.
Clark, U.S. Pat. Nos. 3,976,497 and 3,986,997, and Ubersax, U.S. Pat. No. 4,177,315, describe coating compositions which comprise a dispersion of colloidal silica in a lower aliphatic alcohol-water solution of the partial condensate of a silanol of the formula RSi(OH).sub.3, such compositions containing 10 to 50 weight percent solids of which 10 to 70 weight percent is colloidal silica and 30 to 90 weight percent is the partial condensate. The abrasion resistance of the compositions described in Clark and Ubersax is imparted primarily by the colloidal silica. There is no suggestion in either Clark or Ubersax that any additional particles, let alone colloidal antimony oxide, may be added for improved abrasion resistance.
A coating composition similar to those described in the Clark and Ubersax patents is disclosed in Baney, U.S. Pat. No. 4,275,118. Baney describes an unpigmented coating composition which contains a dispersion of colloidal silica and colloidal titania in a water-alcohol solution of the partial condensate of a silanol having the formula RSi(OH).sub.3. The colloidal titanium dioxide is utilized as an ultraviolet absorbing means which is easily incorporated into the composition and is not itself degraded as the coating weathers. However, Baney clearly shows that the addition of the titanium dioxide actually decreases the abrasion resistance of the resulting coatings.